The present invention generally relates to delivery systems for cryogenic liquids and, more particularly, to an apparatus for use therewith to permit accurate metering of the quantity of a cryogenic liquid, such as liquid natural gas, delivered to a use device.
Liquid natural gas (LNG) is one alternative energy source which is domestically available, environmentally safe and plentiful when compared to oil. Because one of the largest uses of oil is to power motor vehicles, great efforts have been made to develop natural gas engines and systems for delivering natural gas from a storage tank to the engines.
A major problem associated with the delivery of a liquid cryogen from a storage tank to a use device is two phase flow wherein the stream delivered to the use device includes both liquid and gas. This problem is of particular importance in the context of accurately measuring the quantity of LNG delivered to a vehicle for use as fuel. Heat causes the LNG to gasify thereby producing two phase flow and entraining vapor in the fuel delivered to the vehicle. This problem makes measuring the quantity of LNG delivered very difficult. Prior efforts to solve this problem, such as disclosed in U.S. Pat. No. 5,360,139, include circulating the product either continuously or before each delivery to cool down the meter. This introduces considerable heat into the stored liquid causing undersirable pressure increases and likely leads to product venting. Such systems may also provide a path back to the storage tank of the metered product. A leaky valve could return product to the storage tank that the meter has indicated as being delivered to the customer.
Another shortcoming of current systems is inaccurate density determination due to inaccurate temperature measurement.
Therefore, it is desirable to develop an apparatus for use with a cryogenic liquid delivery system to permit the quantity of fuel delivered to a use device to be accurately metered. The present invention overcomes both of these problems resulting in more accurate product metering.